The present invention relates in general to vapor generators and, more particularly, to improved expansion elements for a fossil-fuel boiler casing connected to a membrane tube wall which resists tearing and distortion due to thermal expansion and contraction.
Most modern vapor generators have water-cooled wall panels known as membrane walls. The membrane walls are composed of rows of tubes, laterally spaced on centers wider than the tube diameter, which are connected by flat metal bars known as membranes. Thses bars are positioned 180.degree. apart on the outside surface along the length of the tubes and continuously welded thereto and to adjacent tubes so as to form a continuous wall surface comprising an alternate succession of elongated circular tube surfaces and elongated flat membrane surfaces that are stepped inwardly of the outermost surface of the tubes within the intervening spaces between the tubes. The welds may be formed by various known means and are usually formed on both sides of the membrane wall.
Metallic sheets or plates, known as casing, are attached to the membrane wall to form a gas-tight cased enclosure, for example, such as a windbox for housing the vapor generator's burners and for distribution of combustion air. It is essential that the casing be connected to the membrane wall in a gas-tight manner.
Connection of the casing and membrane wall is accomplished by the placement of short blocks of filler bar in the spaces between the tubes, adjacent to the outer side of the membrane bars. An arrangement in which filler bars are welded in place between tubes is disclosed, for example, in U.S. Pat. No. 3,357,408. The filler bars are seal welded between adjacent tubes to provide a flush, continuous surface transversely across the tubes and the intervening spaces. The casing, in turn, is seal welded to the filler bars and tubes. This type of structural arrangement stiffens the tubes and restricts the ability of the tubes to expand and contract.
During operation of the vapor generator, considerable temperature differences between the membrane wall and the casing subject them to different amounts of thermal expansion. The stiffened arrangement, moveover, has been found to be quite sensitive to accelerated temperature excursions, such as are experienced in some cycling vapor generators during changes in the vapor generator's operating conditions, and may eventually lead to excessive thermal stresses and resultant tube failures.
U.S. Pat. No. 4,538,550 discloses a seal attachment for a casing. No expansion element is used in this structure.
U.S. Pat. No. 2,655,238 discloses a fluid heater casing for a vapor generator or boiler, comprising a plurality of flat plates which overlie an insulation layer which in turn, covers the furnace tube walls. To avoid thermal expansion, the edges of the plate do not touch each other.
U.S. Pat. No. 2,736,400 discloses a wall construction utilizing a plurality of rectangular, flanged seal pans that are seal welded together at their edges. The flaired construction of abutting seal pans provides a V-type expansion joint. A staggered arrangement of the seal pans avoids cumulative thermal expansion effects.
U.S. Pat. No. 3,310,038 discloses a gap sealing structure for components that are subject to three dimensional changes in position. A compensator element which has several possible cross sections including a C-shaped or omega-shaped cross section, is provided. The termination of the compensator is co-extensive with the attachment to the furnace tube wall and does not provide any special arrangement to avoid tearing or distortion of the casing at the termination of the compensator.
U.S. Pat. No. 3,479,994 and U.S. Pat. No. 3,592,171 disclose corner constructions for wrap-around windboxes and for a pressurized penthouse of a boiler, respectively. The use of an expansion joint having an accordion pleated or V-shaped cover plate over an arcuate slot is shown in U.S. Pat. No. 3,479,994. U.S. Pat. No. 3,592,171 uses an expansion joint having a U-shaped cross section to cover a slot in a plate. Here again, however, the slot and the expansion joint terminate at the same location.
U.S. Pat. No. 3,793,991 discloses an expansion plate or sheet having a plurality of trough-like depressions extending substantially the full distance between the two adjacent walls it connects. The device is particularly adapted for use in the connection between the lower headers at the outlet of an economizer of a boiler, and the hopper.
None of the aforementioned patents, however, teach the usefulness of a specially constructed termination for the expansion element of a casing which resists tearing and distortion of the casing at the attachment area to the tube wall.